This invention relates to a circuit board of high reliability and particularly to the surface treatment of a copper wiring suitable for securing a high adhesiveness between the copper wiring and resin on a circuit board.
In order to increase the adhesiveness between the copper wiring and the resin, it has been often used to roughen the surface of the copper wiring to increase the anchoring action of the copper toward the resin and also to increase the chemical bonding of copper to the resin due to the coordinate bonding or hydrogen bonding. In order to increase the anchoring action of copper to the resin, it is necessary that the projections of copper on the roughened surface of copper wiring are mechanically and tightly embedded in the resin and the projections themselves are also mechanically strong.
As a roughening treatment of the surface of a copper wiring to increase the adhesiveness between the copper wiring and the resin on a multi-layered, printed board, U.S. Pat. No. 4,642,161 discloses a process which comprises roughening the surface of a copper wiring by etching, forming a copper oxide layer thereon, and reducing the copper oxide layer, thereby obtaining a chemically stable, finely roughened layer surface. However, the thus obtained roughened layer is poor in the chemical activity and the projections themselves are not enough in the mechanical strength Thus, the roughened layer is not sufficient for producing a multi-layered, printed board of much higher reliability.
Independently of the U.S. Pat. No. 4,642,161, Japanese Patent Application Kokai (Laid-open) No. 52-79271 mentions roughening of a copper wiring by electroless plating or electroless plating on a surface-roughened copper wiring, thereby increasing the adhesiveness between the copper wiring and the resin, but fails to disclose detailed conditions for carrying out the electroless plating in Examples.
The Japanese Patent Application Kokai (Laid-Open) No. 52-1-59271 also mentions electroless plating of nickel or other metals on a copper wiring by reduction and formation of rather porous metal layer with projections on the copper wiring by controlling the electroless plating conditions, irrespective of the metal species, but fails to disclose detailed actual conditions for carrying out the electroless plating in Examples. Thus, it is technically quite impossible to produce a desired surface-treated layer.
The adhesiveness between a copper wiring and a resin depends upon the mechanical characteristics, and chemical and physical bondings at the boundary between the copper wiring and the resin. In case of a resin having a poor affinity to the copper wiring, it is difficult to obtain sufficient chemical and physical bondings. Even if the chemical and physical bondings are high, it is necessary to take the coefficient of thermal expansion and the modulus of elasticity of the resin into consideration when there is a possibility for thermal changes. When the resin has a high modulus of elasticity and a more largely different coefficient of thermal expansion than that of a copper wiring, it is difficult to obtain a satisfactory adhesiveness.